Electrospray emitters are commonly used to transfer analytes emerging from capillary or chip format separation techniques into charged species for mass analysis. Thus, electrospray emitters have found use in capillary electrophoresis (CE), capillary electrochromatography (CEC), open capillary electrochromatography (o-CEC), and nanoscale liquid chromatography (nano-LC). Sheathless electrospray and nanospray emitters meet the demands for high sensitivities, low flow rates, and reduced peak broadening effects on separations, whereas widely used and very robust sheath-flow interfaces dilute the eluting analytes with additional liquid and therefore cause a decrease in sensitivity at very low flow rates.
Electrospray is achieved by applying a high potential (between 2 and 5 kV) to the spray tip, relative to the orifice of the mass spectrometer, which represents the counter electrode. In sheathless electrospray emitters the electrical potential is applied to a conductive coating on the outside of the spray tip, which is ideally part of the separation capillary so that it does not introduce dead-volumes or flow distortions. The high electric field strength used for electrospray leads to sputtering of the coating, as well as electrochemical reactions at the interface, mainly electrolysis of water in the positive spray mode. The use of sheathless electrospray emitters is therefore limited by short lifetimes that are typically up to 100 h for standard electrospray and 6 h for nanospray. In addition, gas formation caused by solvent oxidation and corrosion of the conductive metal surface can result in mechanical and oxidative stress on the coating.
Several coating procedures have been described in the literature ranging from vapor deposition of noble metals and application of conductive polymers, to gluing gold particles (“Fairy Dust”) or graphite (“Black Dust” and “Black Jack”) onto fused-silica capillaries. The latter methods produce very stable sheathless electrospray ionization (ESI) emitters with exceptional lifetimes of over 300 h of continuous use.
Thin, smooth metal films facilitate the electrospray process by creating a high surface tension at the very small contact area of the eluent with the electrode. Vapor deposition of metals and their combinations, e.g., silver, gold, titanium, and chromium, onto the spray tip is usually achieved in vacuum. This procedure, however, cannot be applied to very delicate capillary columns modified with monolithic materials or wall coatings as stationary phases, which require storage under a buffer solution or solvent to prevent desiccation. Accordingly, there is a need in the art to develop a metal coating process for producing stable electrospray emitters that is compatible with the delicate capillary columns used for electrospray emitters.